(Again, have been looking for this book for a while)
This romance book was based around a poor woman who meets a lord (duke, earl, etc etc) and he takes her in or decides to make her his mistress, something along those lines. She had lived in a apartment type home with several other people, across the hall was a prostitute with an abusive husband or boyfriend or such. Later in the book, after a fight with the hero, she returns to the apartment, pregnant, and finds there was a cholera outbreak in the area. The prostitute dies, and the heroine also catches it. When the hero finally finds her again, she is in a hospital and has lost the baby.
I don't really remember much more than that, except that the heroine MIGHT have had another man interested in her before the hero arrived. Need some help here!

I think this is If You Deceive by Kresley Cole._________________"'Happily ever after, or even just together ever after, is not cheesy,' Wren said. 'It's the noblest, like, the most courageous thing two people can shoot for.'" -- Fangirl by Rainbow Rowell

Yes, I also think it's If You Deceive by Kresley Cole, the third book in the MacCarrick Brothers trilogy. This is one of my all-time favorites books.

Burning vengeance...

Ethan MacCarrick was a heartbreakingly handsome rake until a powerful nobleman ordered him brutally beaten and his face scarred for a crime he didn't commit. Ethan's reprisal -- bankrupting the nobleman and forcing his exile -- does little to appease his wrath. Ten years later, a haughty, mysterious beauty enchants Ethan -- the daughter of his enemy. At last, Ethan will have the revenge he's craved; he'll promise her marriage, seduce her, then cast her aside.

Bitter hardships...

When Madeleine van Rowen's family was suddenly plunged into destitution and dishonor, she steeled herself against further heartache. She never weakened, never trusted, until a towering, scarred Highlander relentlessly pursues her, breaking down her defenses.

At what price forgiveness?

The passion between them burns hotter than Ethan's fury, and soon he finds he can't let her go. But when Madeleine uncovers the truth about him, can Ethan convince her to accept all he now offers -- when he once destroyed everything she had?